October 16, 2012

1. Overview and Project Motivation
2. Scientific Theory and Radiometry
3. Overview of Polar Cube Payload and Physical
Constraints
4. Radio Frequency Receiver
5. Diplexer
6. Intermediate Frequency System
7. Operational Calibration
8. Digital Board
9. Sampling and Power Constraints
10.Milestones
11.Acknowledgements

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Our payload will measure air temperature at various altitudes with a surface spot size of 15 km and temperature accurate to within 2 K o This level of accuracy allows for the data to be used for meaningful weather prediction
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Our primary science mission is to track polar ice retreat
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The payload will be smaller, lighter, and cheaper than current weather satellites
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A reproducible satellite bus allows for the possibility of a fleet of low cost satellites which will increase the amount of useful data collected

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The ALL-STAR satellite bus, which will carry our payload, was designed by an interdisciplinary engineering team working with the Colorado Space
Grant Consortium
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We will work closely with members of COSGC to integrate our payload with their satellite bus.
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Team TeamWork, an EE capstone group from fall 2011, created a proof of concept radiometer, which we will further miniaturize and integrate with the ALL-STAR bus
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Scientific guidance regarding radiometry and remote sensing will be provided by Dr. Al Gasiewski

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Airborne compounds selectively pass certain bandwidths of electromagnetic radiation while attenuating others
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PolarCube will use 118.75
GHz, a resonance frequency of diatomic oxygen, as a center frequency o This frequency was chosen because it is stable and well known
Courtesy of Dr. Gasiewski

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PolarCube will measure 8 channels above the center frequency
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Each channel has a corresponding weighting function which gives that channel's weight on atmospheric temperature as a function of altitude
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This graph shows the weighting function of all 8 channels at nadir (facing downwards at the earth)
Courtesy of Lavanya Periasamy , University of Colorado

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—
3U CubeSat specifications to be connected to the
•
•
• primary ALL-STAR system housing
—
—
—
3
—
• structure)
— center of the payload.

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•
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The IF circuit boards will be assembled into the two sides of the PolarCube payload.
The diplexer connected directly with the IF higher frequency circuit board, located in the right hand box.
The IF low frequency board will be located in the left hand box
Courtesy of Space Grant Consortium, University of Colorado
*

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Amplifiers:
two signals out of the diplexer will go through two different amplifiers. They will amplify the power of the two different frequencies.
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Two splitters: equally divide the power to 4, then feed to each individual channel
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Eight filters: the band pass filters will choose the frequency interval that we want.
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Diode detector & RC circuit: transform the signal to DC.
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8 Amplifiers: amplify the DC signal.
Courtesy of Space Grant Consortium, University of Colorado
*

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Diplexer design
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Amplifier and power splitter for both signal bands
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Diode detector & RC circuit design
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Downsize the PCB boards in order to match the mechanical requirement

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The diplexer splits the signal from RF Receiver into two bands
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The diplexer is required because the amplifiers and splitters after the diplexer are not designed to work with such a wide band of frequencies

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Microstrip is a electrical transmission line made using a printed circuit board •
The bottom layer is a conductive ground plane covered by a dielectric substrate. Above this is a conductive trace used for the signal

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Microstrip transmission lines are commonly used for microwave-frequency signals to reduce crosstalk •
We will use them for both signal traces and implementing low-pass and high-pass filters

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Bandwidth: 0.1GHz~1.3GHz
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Cutoff Frequency: 1.3GHz
Courtesy of Kyuil Hwang

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Bandwidth: 1.3GHz~6.5GHz
Courtesy of Kyuil Hwang

Va(t)
Signal detection
Vb(t)
• Bandpass Filter(BPF CF/BW MHz)
CF: the center frequency of the frequency channel
BW: the bandwidth of the frequency channel
• The temperature weighting function
• On the frequency domain: The BPF will ideally pass all frequency between two limits frequencies, which is the lower and upper cutoff frequency, and bar all other signal with frequency that does not within the limits.

V
VB
(t)
Signal detection
V
VD
(t)=V
VB
2 (t)
• Diode detection: the function of the diode detector is the square law, which means the output voltage has the square value of the input voltage.
• Transformation:
• Output signal: in the frequency domain, we will have the convolution of Fourier transform of the signal by itself.
Slides from ECEN5254 Remote Sensing Signals and Systems

Signal detection
• RC circuit: it has the function as a lowpass filter. It will ideally only pass all the frequency between 0 and the particular limit frequency.
• Output signal: the out signal is filtered to a lower frequency with a bandwidth 1/τ, which is the cutoff frequency of the RC circuit. The signal with frequency that’s not within the bandwidth may also pass through it a little bit, and we call it the roll off signal.
• Comparison: the dot line is the diode detector output signal, the real line is the
RC circuit output signal. Since 1/τ << B,
After the RC circuit, our signal frequency range will reduce to KHz.

Signal detection
• Signal processing: the input and output signal of the whole intermediate frequency circuit board will look basically like the two graphs. And the three main function, which are BPF, diode detector, and the LPF(RC circuit) will reduce the frequency signal.

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RF hardware is susceptible to variance in readings due to temperature change making calibration necessary
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We will use the Y-factor method for calibration, which requires measuring two known temperatures: o The mixer using thermistors o Cold space by rotating the lens towards zenith
(away from earth)
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Noise will be be added and removed from the system using a PIN diode o the ratio of the system with and without noise will be used to determine the system's gain due to temperature

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Each rotation of the spacecraft will produce a single line of horizontal resolution--termed a raster •
Movement across the surface of the Earth to generate sequential raster lines will be a consequence of orbiting the Earth •
Rotation rate of the spacecraft is determined by mechanical constraints of the bus and science mission
• requirements
Rotation rate will be tentatively one rotation per second •
Antenna beamwidth determines the spot size on the ground, and will be approximately 15 km 2

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ADCs will have 24 bits of output •
Samples will be averaged to further account for noise
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Range of ADC input voltages: 0-2.5V
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Number of records in raster: 52
Data rate: 530 kbps > ALL-STAR data rate

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Average 32 digitized samples for each channel to provide more accurate measurements and further account for noise
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For diagnostic purposes, calculate standard deviation of sample results in software and provide to ground when commanded to v
0
= <v
0
> + <noise>
σ/<v
0
> = 1/sqrt(Bτ) ≈ 0.01
τ ≈ 0.1 ms
Courtesy of Space Grant Consortium, University of Colorado: ALL-STAR
ICD

Power Supply •
Nominal voltages: 3.3 and 12 volt rails and unregulated battery at 8.4 volts •
Power: 4 watts with occasional peak power draw of 30
W for 15 minutes once every orbit (when the COM system does not require it)
Communication System •
Information Uplink Rate: 9.6 kbps
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Information Downlink Rate: 250 kbps •
Downlink Visibility Time: 9 min
Total Downlink Potential Per Orbit: approx. 16.88 MB

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Decide on A/Ds and a microprocessor
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Design the PCB for the digital board
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Implement science data format
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Write software to handle commands from ALL-STAR, control data transfer to ALL-STAR
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Implement software to display/analyze data from
PolarCube after its reception on the ground

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Digital and RF/IF implementation, prototyping - Begin week of
October 22nd o Begin prototyping designs for diplexer and IF carriers
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Digital design milestone - Begin week of November 29th o Demonstrate functional software and digital hardware correctly operating in conjunction with previous project radiometry prototype
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Final architecture and requirements specification - Week of January
14th o Finalize ALL-STAR ICD and digital system commands and modes of operation
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Integration and testing - mid-February through mid-March o Test digital and RF/IF systems for correct functionality, appropriate system interoperability, and performance

Special Thanks to:
Dr. Gasiewski
Brian Sanders and the team at Space Grant
Lavanya Periasamy
Kyuil Hwang